text:
Is it possible to control the inlet Mach no. of the engine during flight?
While calculating the off design conditions of the engine, we need to know the off design characteristics, such as inlet additive drag. In the book Mechanics of Flight, by Warren F. Phillips, its expression has been derived as a relation between free stream and inlet Mach no. Further while calculating the drag,in one of the example (2.6.1), the inlet Mach no. has been assumed constant at 0.6 whereas the flight is accelerating from Mach 0 to its design Mach no.
This implies that the inlet Mach no. has been somehow achieved as 0.6 even at takeoff, is it practically possible? If not, then how is the drag actually calculated during flight with varying free stream and inlet Mach numbers?

Remember, that the compressor sucks in the air in the static case.

The inlets of supersonic aircraft routinely control (reduce) the Mach number of the engine (at compressor inlet) during flight. This is done using either a 2D ramp type inlet (like Concorde) or a center body (like Mig-21).
These inlets convert the incoming supersonic flow to subsonic flow by forming shock waves in the path of the airflow.

Source: sturgeonshouse.ipbhost.com
In subsonic conditions, the inlets perform two functions:

At cruise condition, diffusion of the free-stream flow to the compressor inlet condition.
At takeoff, acceleration of static air to the compressor inlet condition.

So basically, the inlet design can be optimized such that the Mach number is constant (within some variation) over the engine operational range.
In your case, however, I think that it is a simplifying assumption made for the problem.

So at high speed, the concord had a non zero bypass ratio ? Or maybe the term does not apply to non turbofan engines ?

@Antzi, the air dumped overboard does not pass through the compressor and is not accelerated, so it does not count as bypass.


text:
What causes the reduction in speed across the shockwave?
Wherever I read about shock waves, it is mentioned that there is a reduction in speed across the shock wave.
Can someone please explain what happens at the molecular level, i.e. the physics of the problem?

Hello Manish, welcome at Aviation.SE.

Yup. Looking forward to asking a lot of questions. :D

@Manish - you accepted the answer but did not upvoted it?

@PeterMasiar Probably at that time, I was not allowed to upvote.

And you still consider Trebia's answer better? For me, talking about air molecules in supersonic  "to know" or "not to know" about the obstacle sounds like a magic, IOW does not help at all. Compared to Peter Kämpf's answer, he talks about the collisions between the molecules, which is physics and not magic.

@PeterMasiar What I like about Trebia's answer is after reading it I understand better than I understood before I read it. Peter seems to be making some assumptions about my level of understanding. Ex.: "In case of the oblique shock this is clearly the bend in the wall contour which forces the flow to change direction." That's not clear to me at all. "The index 1 denotes conditions ahead of the shock, and 2 those downstream of the shock." Isn't ahead of the shock and downstream of the shock the same place?

@ToddWilcox -  "Ahead of the shock" is "upstream". Air flows from the upstream, through the shock, and then downstream. But I get your suggestion that Peter Kämpf's explanation might be too technical. But Trebia Project's explanation feel too magical to me - like the lies we say to children when we don't want to explain something complicated. I guess it depends of the intended audience, but I get your point. I am far from expert myself too. :-)

@ToddWilcox - I just noted that  Peter Kämpf's explanation also uses anthropomorphism: "boundary between blissfully unaware molecules at ambient pressure ahead of the shock and their bruised brethren downstream which have just crossed that boundary" - I guess for us humans it is just easier to relate to what happens in the shock if described that way.

The mechanism is not as complicated as might seem in a first glance. First thing, what's a supersonic flow? Is a flow where the speed of the air is higher that Mach > 1 and if we are talking about air that is coming from an upstream subsonic flow the air will be at lower pressure (that is what is happening on the upper surface of a wing).
So, high speed air with low pressure that tries to move along the wing up to the end of the wing (the trailing edge), but at the end of the wing the air is at the upstream subsonic pressure... so, air at a higher presure usually tries to go into low pressure air... but the supersonic air has a lot of inertia.
Finally we end up having air of high speed very difficult to stop... we are talking about very high speeds and the only way the air can adapt is in a very small space with a shock wave.
That is the mechanism that is called "shock wave in adverse pressure gradient".
There is another mechanism that has been described above, supersonic flow agaisnt obstacles.
Imagine that we have a supersonic flow moving towards a step, or a ramp... obviously the air has again the same high inertia, is supersonic!!! But the air "does not know" that there is a ramp. Why not? Because the information within fluid moves at the speed of sound, the molecules of the air that bound of the ramp are not able to reach fast enough the other molecules fast enough.
So... a high inertia flow moves towards a ramp and suddently notices a ramp that the flow was not adapting before and needs to adapt!!! So does it suddently in a very similar way as before.
Another way to understand the mechanism is by looking at the waves that are created when we throw a stone into water. What happens when they approach to the shore? They continue as there is no shore until, suddently they notice it and they die or bound on it. Is the same mechanism (actually the same!!! supersonic moves in "waves").
Finally, just to remark that shock wave has a finite size, is really small but is not a 0 thickness surface. Notice that we have an area where a high speed is transformed into low speed in a very small space. There the viscosity plays some effect.

just to clarify: from a stationary air's perspective, it bumped into the air at the end of the wing. the air at the end of the wing was not stationary, it was accelerated by the wing to travel with the wing already. If the stationary air bumped into the air brought by the end of the wing abrupt enough, you will have a shock wave.

If you look what causes the shock, there must be an obstruction at the root of it. See the sketch below for a straight and an oblique compression shock:

In case of the straight shock this can be a blunt body or an intake filled with slower moving air of higher pressure. In case of the oblique shock this is clearly the bend in the wall contour which forces the flow to change direction.
The index 1 denotes conditions ahead of the shock, and 2 those downstream of the shock. For weak straight shocks the product of the speed ahead of the shock $v_1$ and the speed past the shock $v_2$ equals the square of the speed of sound: $$v_1\cdot v_2 = a^2$$
If we call the local Mach number $Ma$, and if $Ma_1 > 1$, then $Ma_2$ must be smaller than 1, so the flow is always decelerated to subsonic speed by a straight shock. The same holds for the normal component of an oblique shock: It also becomes subsonic. Since the total energy of the gas doesn't change, its pressure, density and temperature rise when it is decelerated.
The incremental pressure change $\delta p$ due to the bend with an incremental angle of $\delta\vartheta$, expressed in terms of the undisturbed flow with the index $\infty$, is proportional to the change in the streamlines:
$$\delta p = -\frac{\rho_{\infty}\cdot v^2_{\infty}}{\sqrt{Ma^2_{\infty} - 1}}\cdot\delta\vartheta$$
Gas pressure on a molecular level is the number and severity of particle collisions. The air molecules experience more collisions on the downstream side of the shock, since air pressure is higher there. The average  direction of the additional collisions is indeed orthogonal to the shock, because it is the boundary between blissfully unaware molecules at ambient pressure ahead of the shock and their bruised brethren downstream which have just crossed that boundary. Once a molecule has passed the shock, the collisions are coming again equally from all sides and its speed does not change any more.
In subsonic speeds this pressure change can radiate in all directions and becomes a shallow pressure gradient. In supersonic speed, no information about the upcoming pressure change can travel forward, so the change is concentrated in the shock front.

Your answer is pretty good, but can you please give me the reference of how we got v1⋅v2=a^2

@Manish: NACA Report 1135, look for Prandtl's relation.

$Ma_1$ and $Ma_2$ are introduced without reference - i.e., what are they?

@ToddWilcox: Well, they do have a reference here: "The index 1 denotes conditions ahead of the shock, and 2 those downstream of the shock". They are the Mach equivalent to v$_1$ and v$_2$, ie. the Mach number before and after the shock.

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text:
What does it mean when autoland is fail passive / fail operational?
The below explanation is what I've got from the internet about fail passive and fail operational:

Fail operational - Where you have 3 autopilots engaged, such that any single failure will result in the remaining 2 autopilots being able to safely land the aircraft. That is if there is a failure autoland remains operational.
Fail Passive - Where you have 2 autopilots engaged, such that a single autopilot failure will result in the disengagement of both autopilots with no significant deviation from the established flight path. In this case the pilot must manually take control and go-around/land. (avsim.com)

Can someone confirm whether the above explanation is correct?
I've read that the A320's Autoland is fail operational but that does not match with the above explanation (since the 320 only has 2 AP's hence making it fail passive).
And also (if possible) can someone give examples of aircraft whose Autoland is fail operational/passive?

As far as I know the A320 has two autopilots each of which has two lanes. So essentially it has four autopilots and not 3 or 2. One autopilot lane pair with two lanes is already fail passive. Engage the second pair of lanes (AP2) and you have dual fail passive systems. Now you can also monitor and compare the AP1 and AP2 signals of each lane... If one lane fails you immediately can vote it out, fail and keep operating (fail operational)
If you only had two lanes you would never know which one was correct, so you disable both, fail passive.

The quoted explanation about "fail operational" and "fail passive" is correct, in that "fail operational" means the system will continue to function after an failure, and "fail passive" means the system will not misbehave after an failure.
The exact number of autopilots required to make this work, however, is debatable; and much depends on how you define it.
Let's take the Boeing 777 because it is easy to begin with. It has three autopilots. It will say "LAND 3" or "LAND 2" respectively. In a system of three, a failure can be detected if one disagrees with the other two. In a system of two, it is uncertain which one has failed, so all units come offline.
Now here comes the interesting part: what if I can distinguish a failed unit from a functional unit, without using another functional unit? In other words, what if I have something that can monitor the existing units, but is incapable of functioning the same as one of the monitored units on its own?
That is how fail operational can be implemented with two autopilots. For example, the Flight Management and Guidance System (FMGS) on the Airbus A320 can switch from "Dual Mode" to "Single Mode". The FCOM does not detail how it does that. (A320 FCOM - Aircraft Systems - Auto Flight - General)
But the FCOM does state a "CAT 3 DUAL" automatic landing is fail operational

the remaining part of the automatic system allows the aircraft to
  complete the approach, flare and landing

while a "CAT 3 SINGLE" landing is fail passive:

there is no significant out-of-trim condition or deviation of flight
  path or attitude, bu the landing is not completed automatically

 A320 FCOM - Procedures - Normal Procedures - Landing Categories
The Boeing 737 has 3 inertial sources but only 2 autopilots. It implements fail operational on the inertial sources but fail passive on the autopilots.

If a failure is detected, the flight controls respond to the A/P
  commanding the lesser control movement. If a failure occurs in one
  A/P, the failed channel is counteracted by the second channel such
  that both A/Ps disconnect with minimal airplane maneuvering and with
  aural and visual warnings to the pilot.

Boeing 737 FCOM volume 2 - Automatic Flight - System Description

Is the A320 autoland fail operational/fail passive?

Ok, no prob. Because if the A320 is fail passive, the above explanation would make more sense.

@Bianfable but then how do you know which one has failed? It works because in a system of three, you can kick the one that disagrees with the other two. In a system of two, you don't know which one is good, so you must kick both of them.

@Bianfable I just pulled out the B737 manual for reference. On LAND 3 it has two autopilots but three inertial sources. The manual also describes LAND 3 as fail-operational and LAND 2 as fail-passive. The relevant chapter is "Automatic Flight" in FCOM v2.

@kevin Each autopilot should have a monitoring channel. If the monitoring channel disagrees with the command channel, it's considered failed.

With only one autopilot left, is it still possible to check for every possible failure of it? As in, there no longer is a secondary one to validate against and I don't know whether the capabilities of the monitor go far enough.

Definitions provided by the FAA can be found in AC 120-28D - Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout. From the document we find for Fail Operational:

4.3.2. Fail Operational Category III Operations. A Fail Operational System is a system which after failure of any single
component, is capable of completing an approach, flare and touchdown,
or approach, flare, touchdown and rollout by using the remaining
operating elements of the Fail Operational system. The failure effects
of single components of the system, airplane or equipment external to
the airplane which could have an effect on touchdown or rollout
performance must be considered when evaluating Fail Operational
systems.

With the following examples:

The following are typical arrangements which may be acceptable for
Fail Operational Systems:

Two or more monitored fail passive autopilots or integrated autopilot flight director systems each with dual channels making up an
automatic fail operational system designed so that at least one
autoflight system remains operative after the failure of one system,
and the failed system is not used or cannot cause unacceptable
autoflight system performance.
Note: Following a failure with this
configuration, it is not intended that a landing be continued with
flight director alone, unless a successful Proof of Concept
demonstration has been completed.
Three autopilots or integrated autopilot flight director systems designed so that at least two remain operative after failure to permit
comparison and provide necessary monitoring and protection while
continuing to a landing.
A monitored fail passive automatic flight control system with automatic landing capability to touchdown and rollout, if applicable,
plus an independent and adequately failure protected manual flight
guidance system, suitable for landing and rollout with guidance
provided for the flying pilot and monitoring displays for the
non-flying pilot. A proof of concept demonstration would be necessary
for this arrangement.
Two independent and adequately monitored manual flight guidance systems with independent displays for the pilot flying and the pilot
not flying, each capable of supporting a landing and rollout. A proof
of concept demonstration would be necessary for this arrangement.

The key to fail operational is the ability to continue the operation after a single fault. It's about the capability, not the solution that provides the capability.
And for Fail Passive:

4.3.4. Fail Passive Category III Operations. A Fail Passive System is a system which  in the event of a failure, causes no significant
deviation of aircraft flight path or attitude. The capability to
continue the operation may be lost and an alternate course of action
(e.g., a missed approach) may be required.

With the following examples:

Typical arrangements which may be used to meet the requirements for
Category III fail passive operations using a 50 ft. Decision Height
include the following:

A single monitored automatic flight control system with automatic landing capability.
A fail operational automatic flight control system with automatic landing which has reverted to a Fail Passive configuration or has been
dispatched in a fail-passive configuration.
A monitored flight guidance system (e.g., HUD) designed for manual control by the pilot flying, and for monitoring by the pilot not
flying. Aircraft intended for Fail Passive Category III operations
should have aircraft systems which meet the criteria specified in
Appendix 3. Aircraft previously demonstrated to meet earlier Fail
Passive criteria may continue to operate using Category III minima in
accordance with approved operation specifications.

The key to Fail Passive is that it causes no hazard even though you may not be able to continue the approach.

I don't think the quote you gave is correct. This might be the case for some aircraft (like the Boeing 777 with 3 autopilots), but not in general. A more general definition is:

Autoland systems are normally designated Fail Operational or Fail Passive.

A Fail Operational system must have at least two autopilots engaged for the approach. The failure of one autopilot will still allow an autoland to be carried out. This allows a “no decision height” approach to be conducted.
A Fail Passive system is normally associated with a single autopilot approach. In this case, failure of the autopilot will not result in any immediate deviation from the desired flight path; however, the pilot flying must immediately assume control of the aircraft and, unless he has sufficient visual reference to land, carry out a missed approach. The lowest allowable decision altitude (DA) for a fail passive system is normally 50’.

(SKYbrary - Autoland - System Safety)
So 2 autpilots are enough for a Fail Operational system. I cannot say about the A320 specifically, but the Boeing 737 has a fail operational autoland mode with just 2 autopilots:

Fail-Operational Autoland Status Annunciations
The following annunciations provide the flight crew with autoland system mode
  and status:

LAND 3 – two autopilots, three inertial sources, and the associated
  sensors are operating normally for an automatic landing and rollout.
LAND 2 – a failure has occurred above Alert Height and redundancy is
  reduced; but the autoland system is still capable of making an automatic
  landing and rollout.
NO AUTOLAND – the system is unable to make an automatic landing.

With a LAND 3 (fail-operational) indication, the autoland system level of
  redundancy is such that a single fault cannot prevent the autopilot system from
  making an automatic landing.
With a LAND 2 (fail passive) indication, the level of redundancy is such that a
  single fault cannot cause a significant deviation from the flight path.

(Boeing 737 NG FCOM v2 - Automatic Flight - System Description)

Let us look at the statements and see how right or wrong they are.

Statement 1:
Fail operational - Where you have 3 autopilots engaged, such that any single failure will result in the remaining 2 autopilots being
able to safely land the aircraft. That is if there is a failure
autoland remains operational.

First of all, this statement is not wrong. It is as a matter of correct. Yes, if an aircraft has 3 autopilots, it will remain fail operational if one of them malfunctions during an automatic landing. But you do not need 3 autopilots to achieve fail operational capability. If you want to have fail operational capability with two autopilots, you need two fully independent auto pilot performance monitoring systems. All Airbus aircraft starting from the A320 have two autopilots and they all have fail operational automatic landing systems.

Statement 2:
Fail Passive - Where you have 2 autopilots engaged, such that a single autopilot failure will result in the disengagement of both
autopilots with no significant deviation from the established flight
path. In this case the pilot must manually take control and
go-around/land.

This statement is wrong. With two autopilots engaged, if a single autopilot were to fail, the system becomes fail passive. It is not necessary for the other autopilot to disengage. And in some certain cases, the aircraft can even perform an automatic landing with a single autopilot (in this case it remains fail operational). I will explain how it works later on.
Here are the JAA (current EASA) definitions of both the fail operational and fail passive.

FAIL OPERATIONAL
An automatic landing system is fail-operational if, in the event of a
failure below alert height, the approach, the flare and landing can be
completed by the remaining part of the automatic system. In the event
of failure, the automatic landing system will operate as a
fail-passive system.
FAIL PASSIVE
An automatic landing system is fail-passive if, in the event of a
failure, there is no significant out-of-trim condition or deviation of
flight path or attitude but the landing is not completed
automatically. For a fail-passive automatic landing system the pilot
assumes control of the aircraft after a failure.

If we look at the definition for fail operational, you can see that it does not say anything about 3 autopilots. But it says something about an alert height. It says below alert height, a system is fail operational if after a failure in the automatic system, an aircraft is still able to perform an automatic landing with the remaining part of the automatics system. So, what is this alert height?
According to the Airbus A320 FCTM:

ALERT HEIGHT
The Alert Height (AH) is the height above the runway, based on the
characteristics of the aeroplane and its fail-operational automatic
landing system, above which a CATIII approach would be discontinued
and a missed approach initiated if a failure occurred in one of the
redundant parts of the automatic landing system, or in the relevant
ground equipment.

Alert height is an altitude derived by the aircraft manufacturer during aircraft certification (AH for A320 is 100ft).It is the minimum height at or below which an aircraft is allowed to perform an automatic landing after a failure in its redundant automatic systems. So, for example in an A320, if you were doing an automatic landing with both autopilots engaged and if one of the autopilots fail above the alert height which is 100ft, you will immediately commence a go around.
So, what if one of the autopilots fails below 100ft? The answer for this is it depends. Remember us talking of an autopilot performance monitoring system? In the A320, there is an AUTOLAND warning light on the glare shield of each pilot. This light flickers below 200ft if following conditions are encountered:

Both APs trip off
Excessive beam deviation is sensed
Localizer or glide slope transmitter or receiver fails
RA discrepancy of at least 15 ft is sensed.

The warning light is triggered by the monitoring system, if it detects any of the failures above. So, when below AH, the automatic landing can be continued even if one of the autopilots trip off as long as the AUTOLAND light does not come on. If the AUTOLAND light comes on, you should immediately go around.

Here is a little GIF of the AUTOLAND warning light test of A320 done by me. You can test if it works just by pressing it any time you like (even in flight). If you press it, the light comes on and it goes.

The second statement is correct and not incorrect. You have two autopilots and you compare the outputs to each other. If one autopilot deviates from the other you know one of them is faulty and the other is correct. But you cannot tell which one. Therefor you disable both of them and that leaves you with no autopilots.

The A320 has two autopilots with each two lanes as far as I know. That means it essentially has two fail passive autopilots, or four autopilots in total. The first two lanes monitor each other and the second two lanes monitor each other. When one autopilot detects that their two lanes tell different stories it disengages, leaving you with one pair of lanes remaining. This is fail operational.

@Jan The statement is incorrect because it says, if there is a fault in an autopilot, both the autopilots disengage. It is not that way. If one fails the other takes over and the system becomes fail passive. Even when we do normal approaches it is normal procedure to turn on both auto pilots, so that if one fails, the other immediately takes over. In that case the FMA changes from CAT 3 DUAL to CAT 3 SINGLE.

What I'm saying is that the A320 does not have two autopilots, it has four. So after you loose one pair of autopilot lanes (both engages with a single push button and operating as as single unit) or if you don't turn it on, you still have two computer lanes monitoring each other. Only then you can apply the statement of fail passive.

@Jan The correct way to say it will be when in fail passive (single autopilot on), and if that autopilot also fails, then the aircraft is handed over to the pilot in a safe manner (in trim).

Well if you have one light on (AP1 or AP2) you still have two remaining autopilot lanes engaged. This is a fail passive autopilot (indicated as one autopilot to the pilot but actually two autopilots internally) and if one of these two lanes disagree (failure detected) then the aircraft is handed over to the pilot in a safe manner (in trim)

@Jan if one autopilot disengages the A320 becomes fail passive. It can remain fail operational in this situation only if it is below alert height.

If you had AP1 and AP2 on and one of them detects a failure then you have three autopilot lanes against one faulty autopilot lane. The faulty pair of lanes is removed leaving you with one fully working and fail passive system, yes.

The A320 also does this for its flight control computers as far as I know. Each FCC is comprised of two lanes, which can monitor each other. That way you can make sure that the computer in command does the right thing and no single fault is undetected. Unless there is a rare dual failure happening at the same time :)

@jan Yes. One is the control the other is the monitor. The data needs to be the same for both the channels or within a certain threshold. If not the AP will be disconnected.

Fail operational is a description for an automatic landing autoflight system before any autopilot (AP) fails and is divided into two categories:

Three APs in the aircraft.
Two APs + comparison circuitry (this circuitry can be considered more or less roughly as a third AP).

If one AP fails in either category above (the system now is a fail passive), there is no loss of control but the pilot cannot initiate an autoland because the system is fail passive now.
If a pilot initiated an autoland with every thing operating (fail operational) and then an AP fails the remaining AP (two or one depends on category) will continue to autoland normally.
If we are already in an autoland stage with a loss of a AP (two remaining or one remains operating depends which category) and then a second AP pilot fails pilot immediately takes control.
In any phase of flight if a single AP fails the system changes from fail operational to fail passive regardless how many APs remaining operating.
A pilot cannot initiate (start) an autoland from a fail passive system.
So 2 APs remaining operating in a 3 APs category is a fail passive, similarly if one AP remains operating in a 2 APs category is also a fail passive system.

ALI  ALSALEEM - Ground instructor / Mideast Aviation Academy MEA - Jordan

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text:
Can horizontal stabilizer trim be worked independently (each side)?
I notice most jetliners have a big trim wheel on each side of the center console.  This wheel is black with white stripes, and notably turns on its own via the autopilot.
Two large trim wheels: And obviously, most jetliners have two horizontal stabilizers or stabilators.
Can each side be trimmed independently of each other?  That is, does each big wheel control one stabiliz/ator and can they be unlocked and operated in different directions if need be?

Some also use stabilators.

@aerobot my initial line of thinking is redundancy.

@MichaelHall, was your comment directed at mine or at Harper? I deleted my comment thinking I might expand it into a partial answer, but now there's a floating comment chain.

It was directed at your comment, but it isn't applicable now, so I will delete it.  And eventually, maybe this one too...  ;)

No.  Theoretically you could have the left and right stab move independently so there is still some trim authority if one jams, but would be structurally way more complex and quite heavy, your trim authority would still be cut in half, and there isn't really a strong need to do that from a risk perspective.  There may be aircraft out there that are like that, but most moving stabs are single surfaces usually operated by a big electric jack screw.
The required redundancy is provided by redundant control channels for the trim system, dual drive motors for the screw jack that drives the stab, and a dual load path design in the acme thread and the trunnion (the "nut" part of the jack), and in the attachments that connect the trunnion to the stab and the actuator assembly to the structure.  The only single point of failure would be for the hinge of the stab itself to completely seize, and the risk of that is low enough that it doesn't need to be accounted for in the system architecture.
Two independent stab surfaces would require an attachment to the fuselage or fin that allows independent movement of each surface while cantilevered off the root structure, which would require a pretty heavy and complex center structure for not a lot of gain safety wise.

The first case of flutter occurred on a British WW I bomber and could be cured when the two independent elevators (left and right) were coupled. The flutter case would see both elevators working in opposition, twisting the fuselage. Very few aircraft used independent elevators after that.

I once witnessed (heard, actually) flutter of the vertical tail of a Libelle as it flew overhead at about 500 ft at Vne, or close to it, on a late afternoon when everybody was returning to the field.  Sound like a jet that sailplanes make when they are going fast, then a Bddddddddddddddd... noise for about 1-2 seconds.  Pilot was a tiny bit alarmed with he landed.  I've heard that the almond shaped fuselage cross section of the Libelle made it prone to this sort of thing.

Those early glassfiber designs were prone to flutter. The rudder is still moved by cables instead of pushrods in gliders because the friction of the steel cable in the plastic hoses will dampen rudder flutter. Friction can be tolerated because the pilot's leg strength is sufficient. The tail of the LFU 205 would twist ±30° in a climb (slow and full power) because of rudder flutter. That's another of those early glassfiber birds.

I don't know why one would want split stabilizer/stabilator control in a jet airliner. Not having the same up/down input on both sides would make the plane want to rotate from the tail, fighting any inputs coming from the ailerons and/or wing spoilers.

Some aircraft have a pitch disconnect that allows each yoke to control the elevator on its side separately, in case one side gets jammed.

Splitting the stabilizer would be structurally dangerous if one side is controlled while the other is fixed. One side moving to a position while the other side is floating ( following the air flow) won’t be harmful.
As an exemple on the A320, if one elevator is faulty and the other is active, the faulty one will be floating.
Similarly on the A320 if both elevators are faulty due to computers failures, both elevators will be centered (aligned with the THS), pitch is THS controlled in this case


text:
Which modern day prop (piston) engines allow for inverted flight?
A buddy of mine is looking to buy an airplane with an old radial engine because he says radial engines can fly inverted in aerobatic flying.  What modern day engines (engines that you can buy today), are capable of flying in inverted flight?

Its worth noting, you can still buy new radial engines.

Any “modern day” piston engine can allow for inverted flight if it has the right equipment on it.
The American Champion Citabria with the 7KCAB Inverted Fuel and Oil package along with the American Champion Decathlon are both "Modern Day" aircraft capable of sustained inverted flight.
The major differences between the7GCAA and 7KCAB were in the fuel system and the engine oil system. The engine was replaced with a Lycoming IO-320-E2A of 150 horsepower (110 kW), while a header tank of 1.5 gallons—located beneath the instrument panel—was added to the fuel system. In addition, the carburetor was replaced with a fuel injection system, and a Christen Industries inverted oil system was fitted to the engine. All of these changes were made in order to allow for extended inverted flight, a mode not possible in the earlier models.
disciplesofflight.com/american-champion-decathlon/
You can fly inverted continuously for about two minutes before the fuel in the header tank runs out and the engine falters. Rolling upright will recharge a depleted header tank in about a minute while the faltering becomes a smooth running engine again

I fly competition aerobatics. The only setup I've seen is some flavor of Lycoming engine with extra bits for fuel (flop tube in a header tank) and oil (Christen inverted oil system).
Your friend is correct that some (most? all? -- I don't know) radials can handle inverted flight.  Make sure there's a way to ensure continuous fuel and oil delivery to all the moving parts.
I would ask a lot of questions before turning a Continental engine upside down.  I dimly recall some issue with oil getting past push rods that makes them unsuitable.  (Please note this is not a slam on Continental - they're great engines for upright flight, but I don't know if that fish will climb a tree very well)

Most engines can handle inverted flight just fine, but it is the fuel system (gravity-fed fuel and/or carburetor) that inhibits the airplane from doing so because of their need to maintain a positive G-force to enable operation.

This is a bit incomplete as most engines can not operate inverted. Most engines (in their stock configuration) in inverted flight will suffer oil starvation or improper oil flow as well as the fuel feed problems. Lycoming does make an inverted oil sump package that can be added to some engines you can find details in section 3 here. I belive similar systems are also available for continental engines.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How does the Harrier avoid engine damage when operating from dirt fields?
The Harrier V/STOVL jet was designed in order for its users to be able to retain a functional air force even with all conventional airports destroyed; to this end, it can take off from, and land in, essentially any space large enough for it to fit into, potentially including unpaved dirt pads.
Unlike most jets designed to be capable of rough-field operations, the Harrier has no mechanism for preventing engine dirt ingestion, and, indeed, ingests considerable quantities of dirt and dust when operating from dirt fields:

(Image by @KeithS here at AvSE.)
How does the Harrier avoid engine damage from dirt ingestion during rough-field operations?

According to this RFP FOD ingestion for the AV-8B is a major problem. I'm guessing that they don't "avoid damage" so much as "deal with it when it happens".

I imagine putting the intakes so far forward is one way.

From accounts I have heard of military operation of jet engines, running engines in a manner that shortens time between overhauls is not uncommon.

@acpilot: And yet they still suck in lots of dirt, as shown in the picture.

@Sean But would they ingest more dirt or less dirt if located further aft?

@acpilot, fore and aft doesn't have any effect, it's proximity to the ground and suction power.

How does the Harrier avoid engine damage from dirt ingestion during rough-field operations?

The short answer is that they don't. They avoid using these FOD-laden strips if possible, but doesn't have any kind of active method for reducing FOD damage from debris ingestion.
This is supported by an RFP that NAVAIR put out asking for proposals to reduce FOD for the AV-8B:

The AV-8B currently has the highest FOD damage rate in the U.S. DoD inventory which incurs a cost of more than $100M per year in engine removal, rebuild, and reinstallation, as well as decreased readiness and safety.

The method here is to just deal with it when it happens. The engine is removed, repaired, and replaced when a FOD incident occurs.
The document also says:

To date, NAVAIR has experienced more than eight Class 'A' mishaps in the AV-8B due to FOD

Where mishaps are defined:


text:
What type of aircraft is depicted in the Taylor Swift music video "Look What You Made Me Do"?
It looks like some sort of business jet or small regional airliner, but I can't tell anything more specific than that.  About the only thing I'm certain about is that it's a T-tailed twinjet with rear-mounted engines.
View of the starboard side of the aircraft in question:

Frontal view of the aircraft's empennage:

Apologies for the poor lighting and resolution (especially in the first image).

Look what you made me do. You made me watch a Taylor Swift video!

@TomMcW And yet you Swiftly returned, likely without Taylored clothing.

According to Reddit, it is a Gulfstream G650 with the wings cut off, from a Taylor Swift music video “Look what you made me do”.
It has 8 windows like a G650, but is not a real aircraft.

the engine fans don’t look right
the wing leading edge has non-flush rivets and there are too many of them
the window size, shape, and spacing are wrong
there is no front entry door

And no one would be crazy enough to actually cut the wings off a 65 million dollar aircraft.
I don't think it is CGI because of how poorly some of the details are rendered. CGI should be more accurate.
It’s probably a Hollywood prop left over from some movie.

What do you mean when you say the engine fans don't look right?

@Cloud Feed something like "jet engine intake" to your favorite image search engine. Those blades look at least to me like they would block the air rather than draw it in, by virtue of the almost nonexistent spacing, which would mean that the engines could never function.

@MichaelKjörling: I thought he was referring to the fact that the fanblades on the real G650 are scimitar- or boomerang-shaped, whereas the fanblades on the "Look What You Made Me Do" G650 are completely straight.

What's wrong with the leading edge?  I can't see any difference between the real G650 and the LWYMMD G650 in that regard.

@Sean Possibly. I'd think some clarification from Mike in that regard would be welcome.

One more thing - trying to do that to the wing of a real G650 would flood the hangar with flaming Jet A.

My guess is it's a damaged airframe scheduled to be cut up for scrap. In which case there would be no engines in the nacelles, so the set people would have to make fake fan blades to put in the hole. That would explain odd-looking straight blades instead of scimitar

@TomMcW it could be camera angle/perspective but the tips of the wings in the video appear bent up a lot more sharply than in pictures of G650s, based on that I'd assume it's entirely a prop/mockup not a damaged airframe.

@DanNeely: It looks to me like perspective would probably be enough to account for the difference in winglet angle, but a) we don't get a very good look at the winglets (the camera is focussed on Taylor, who is standing fairly close to the wing root, and the lighting, as I noted earlier, is not exactly the best) and b) I still agree with you that it's most likely a prop or mockup of some sort.

Ah, now I see the issue re: the rivets.

That appears to be a CGI rendering of a jet based upon a Gulfstream G-V or G550.  It appears to have its wings or wingtips clipped off.

@VladimirF despite this probably not being a rendering, that's not a good reason: you can "dance" and do lots of other stuff on renderings, ask on movie.SE

She’s dancing on a green screen rig; the airplane is added digitally in post production.  The record company is NOT going to destroy a $68 million airplane, no matter how much Taylor Swift complains about that

it's utterly commonplace to "dance on" a CGI rendering - for 15? years now.  it's completely trivial to do that.

"A lot is technically possible, but very expensive" you are 30 years out of date, it's completely trivial now, @VladimirF

@CarloFelicione: compositing two real shots with green-screen is not the same thing as CGI.  There's almost certainly a real plane that someone pointed a camera at, not just a fully CGI rendering of a computer model.  I like @TomMcW's guess that it's a damaged airframe that would otherwise be scrap, repurposed as a movie prop.

One may dance on a real aircraft (BTW there is no close up, so it may be a stunt double), then "cut off wings" digitally. Alternatively, they could have used a written off aircraft. Or even a plane under construction.

@VladimirF The chainsaw scene looks really wrong, and doing it on a model seems dangerous, even with a stunt double. CGI is the only sane way to do that.

@Fattie having done some video editing and computer rendering, I can assure you that CGI with human interaction, changing lighting, and photorealistic reflections is far from trivial; though it is much more accessible than even 10 years ago. The official 'behind the scenes' videos show lots of green screen, but never in any scene with the airplane. After watching the airplane scenes at 1/4 speed and checking for rendering artifacts or anything 'too perfect', I'm fairly sure this was a physical model. Could've been rendered, but cheaper and faster to get this level of quality with a prop.

hey @brichins - i work in the business.  cheap as chips.  {particularly compared to the general high cost of a major video for an A-list star}  (Note that, thought I like T. Swift as much as the next guy, I haven't seen the clip, and don't care how it was done!  :) )  Note that merely Joe Khan's fee would totally crush a few dollars for CG.

Whater, lets delete it, its off topic anyway. I resign. I looked on the photos before only because it is not the kind of music I would like to hear.

@Fattie FWIW, dancing on a rendering is much older than that, https://youtu.be/IFGdlSbJAUE?t=3m28s long before the rendering was done by computer.

authors: Vladimir F Героям слава


text:
Why can't the A320's computer-imposed limits be overridden?
Under the investigation section of US Airways Flight 1549, it is noted that

[The pilot] asserted that insufficient credit was given to the A320's fly-by-wire design, by which the pilot uses a side-stick to make control inputs to the flight control computers.
The computers then impose adjustments and limits of their own to keep the plane stable, which the pilot cannot override even in an emergency. This design allowed the pilots of Flight 1549 to concentrate on engine restart and deciding the course, without the burden of manually adjusting the glidepath to reduce the plane's rate of descent.
Sullenberger said that these computer-imposed limits also prevented him from achieving the optimum landing flare for the ditching, which would have softened the impact.

Although this design aids pilots most of the time, why is there not a 'last resort' option to override the computer-imposed limits? It could have been that in this incident, it was the difference between life and death / a 'hard landing' vs a complete hull loss.

The only real answer here can be "That is the design philosophy of that product."  Certainly, a large and complex discussion can be had on the pros and cons of each approach.  But there's no specific, denormalized, single-shot answer.  If Airbus, as it were, were asked this (say in a documentary, history or investigation), all that they could say is "on balance" and "after considering everything" their team decided their way is best.

They're awesome, except they're the sole reason for the injuries which put one of the flight attendants out of her job permanently, the only blemish on what would've been a perfect landing.

How are software limits really different from mechanical limits?  That is, the controls & control surfaces on a purely mechanical design have only so much travel built into them.

@Harper Can you elaborate or link to the information regarding that please?

Computer limits can be overridden on an Airbus as a ‘last resort’, though I have serious doubts that would have helped with the landing on the Hudson incident.
Granted, getting out of Normal Law is not SOP, or a normal procedure..but then again, why would it be?
People complaining about the computer limits on flight controls, are similar to people complaining about traction control on cars....yes, so you floored the acceleration and the car decided to disconect the drive! There is probably one freak accident in a billion that could have been saved by the driver disconecting traction control and then manually skidding to safety, but by-and-large the traction control has saved your butt a hundred times already.
Control laws will protect the aircraft/airframe in the same way. Or like an automatic shift will protect from overreving your engine by shifting up. Can you still blow up an engine on an automatic? probably yes, you just have to try harder....
Really wished pilots who never flew Airbus (or better yet Airbus and Boeing) stop issuing opinions and comparisons on this matter. FBW system in Boeing’s 777 is very similar to Airbus for instance, so the very discussion of Boeing trusts the pilots vs Airbus trusts the computers is starting from a very wrong assumption.

There are 2 philosophies in civil aircraft FBW systems...
* Don't tell the pilot they're exceeding the limit, and don't let them exceed the limit.
* Tell the pilot they're exceeding the limit, but then let them.
As seems to be usual, Airbus and Boeing have differing points of view based upon their experience of working with pilots - guess which is which.
So the answer to your question is simply the manufacturer's philosophy.

@ymb1 Is that similar to alternate-law? Sorry if a stupid question :p

sorry, I have to ask... :-)) what flight limitation is Boeing FBW letting the pilots exceed, that Airbus is not?

@Radu094 For example Boeing 777 allows bank angle beyond 67° - it’s trying to resist very sternly, but will still let you do it. An FBW Airbus won’t.

@Radu094 ... obviously my previous comment applies to Normal Law/Mode only.

@CptReynolds hmm.. fair enough, that's a pretty good point,actually. I guess I could still play devil a bit and point out that bank angle is not a frame limitation per se; It is the loadfactor that is a limitation, (67° bank requires +2G ) and Boeing is also protecting against overload :-)

@Radu094 True. In a way, protecting against overloading makes more sense than limiting bank angle which is only an indirect measure of load and only in a steady level turn. But then, there shouldn’t be any reason to bank that much, anyway... so I feel it’s an academic discussion. The only points I really don’t like in an Airbus (as an engineer) are Alternate Law without low speed protection (very odd design, in my mind) and independent sticks (not helpful for common mental model among crew).

Why there is no switch? Because having it could be dangerous.
The purpose of the computerized system is to reduce pilot's workload. It's achieved by allowing him to pull the yoke all the way back and the computer would give him the highest safe AoA. Adding a switch would increase pilot workload, because he would have to make a very deliberate decision whenever to override the computer or not. Spending time considering cons and pros of overriding the computer diverts pilot's attention from actually flying the plane. Nobody wants that, especially during an emergency.
Mere presence of such option could contribute to the accident, therefore you can't have it.

An emergency can mean that the airplane no longer looks or behaves like the computer assumes it does?

@rackandboneman Yes. But usually the computer knows that and
Sadly, humans may not notice it, stall the plane and kill everybody, like Air France Flight 447. Airplane manufacturers seem to be coming from assumption that having the switch would only cause more AF447 to happen.

The control laws can be downgraded to alternate law, which removes the alpha protection.
The QRH recommends this only in certain situations, and dual engine failure is not one of them. One place where this is recommended in the QRH is for an unreliable airspeed indication. Usually an unreliable airspeed will be recognized by the aircraft and it will automatically switch into alternate law. This is what occurred on AF447. But there are scenarios where the computer may discard the correct data, or none of the airspeed data is valid, yet they match, so the computer doesn't catch it.
Airbus realizes that this may cause incorrect response of the alpha protection, so they have a procedure for dealing with it. They have the pilot switch off the ADR's* one at a time to see if they can isolate the faulty data. If they can't, or none of the ADR's are providing good data, they are instructed to shut down two of the ADR's to force the system into alternate law. The third ADR is to be left on because the stall warning can still function on a single ADR.
 from the A346 QRH:

This doesn't involve pulling circuit breakers. There are buttons on the overhead panel for this. Pulling circuit breakers can negatively affect other control computers, as occurred in QZ8501.
*The ADR (Air Data Reference) is part of the ADIRU (Air Data Inertial Reference Unit) which is the computer that supplies airspeed, Mach number, angle of attack, temperature and barometric altitude data to the FBW system.

For the Airbus fly by wire system there are different so-called control laws which govern the behavior of the aircraft according to the input provided by the pilots. These control laws are determined automatically by the aircraft for different phases of the flight envelope.
This is also the main difference in design philosophy between Boeing and Airbus. Boeing aircraft assume that the pilot is always right and Airbus has a computer between the sticks and control surfaces. In this way they can filter the input from the pilot such that the aircraft does not exceed its designed airframe loads. Another benefit of this system is that they can make their range of aircraft handle in a similar fashion.
Can you override the control laws on an Airbus jet? No, you can't. Why can't you? Simply, Airbus design philosophy is that if the computer can't save the aircraft you can't either. You can however force it by pulling circuit breakers since after an electrical failure you as a pilot have direct control over the aircraft without the computer imposing limits. However, this is not possible during an emergency.
You can read more on the control laws here: Airbus Flight Control Systems
There have been incidents where the computer almost crashed an aircraft by switching too early to a different control law (Lufthansa LH-44):
[Report: Lufthansa A320 at Hamburg on Mar 1st 2008, wing touches runway in cross wind landing][10]

Airbus philosophy is not that if computers cannot save the aircraft, pilots cannot either. That is nonsense, and internet fodder. Control laws can be changed/downgraded even w/out pulling circuit brakers, though procedure is indeed complicated and involves pushing two buttons. Airbus recommends using appropriate amount of automation and taking control when things don’t go as expected

Comments are not for extended discussion; this conversation has been moved to chat. Also, please consider the words you're choosing when engaging in the aviation equivalent of "Apple vs Microsoft".


text:
What part of an aircraft design process is governed by mathematics?
I am well aquainted with the six steps of the engineering process, but what is the six steps of design specifically for aircrafts (if there is one)?
I'm very mathematical, so I would appreciate it if you could include equations that are necessary for each step.
Also, does anyone have a CFD that they would recommend? I'm building models in AutoCad, and am currently working with a CFD called Anasys.
Right now, I understand how the different components of a plane affect its flight, but I am really more interested in understanding how to design a plane using math.  I do a lot of work with rockets, which is governed by a pretty good set of written equations, but I was hoping I could do similar in the field of airplanes.

Maths have their place in pre-design of aircraft, but it is still a design process so creativity has a place as well. Designing a plane is an iterative process, that also includes steps like determining what the plane is used for, what the payload will be, what the range should be, is it allowed to use a runway etc.
Once these factors are determined and honed down, there is a lot of math to be applied to check if the wing can support the weight, if the engines can propel the plane, if the structures can take the static and fatigue load, if the hydraulic pumps are dimensioned correctly, if the control surfaces are dimensioned correctly...
Rockets are relatively easy in that respect. Not taking anything away from the achievement of putting men on the moon, but a rocket has a single mission type, has very little bother with aerodynamics, and does not need to be optimised for decades of safe, economical or mission ready use.
One aspect of aeroplane design where maths & physics rule is aerodynamics, but what's great for aerodynamics is usually not great for structural engineering for instance. So if you would like to focus on maths only, aerodynamics would be a good place to start. Please realise that it is only one aspect of aeroplane design though.
The complete process of aeroplane design is really a lengthy process that does not fit within one answer on this site. I can recommend my goto book for design aspects, Torenbeek. It has 600 pages. Check out Appendix C. on a mathematical method for weight prediction of the wing structure for instance.

You would be surprised how much a rocket is bothered by aerodynamics. Not for nothing is one of the most important points in the trajectory the max q point, that with the highest aero loads.


text:
What is the purpose exactly of a ‘Control Check’?
I recently watched an Air Crash Investigation episode where Qantas flight 32 lost an engine shortly after takeoff. It was revealed that a stud pipe in a Rolls-Royce engine was made incorrectly, causing the pipe to snap and burst through the engine and the aircraft's wing, severing major hydraulic systems, and causing the ECAM to display pages and pages of errors.
On the final approach, the captain announced he wished to perform a ‘control check’.  He said (and this is approximate):

When the aircraft is damaged, you have to certify it is safe to land before you try to land.

I assume this is so that the captain can make the necessary preparations if he feels his aircraft is not safe to land. On flight 32 this was done by moving the stick left and right to simulate lining up with the runway.

Perhaps a better question is back to the post's title.
What are the purposes of in-flight Control Checks?
In my example of Qantas Flight 32, the stick was moved left and right on final approach to simulate lining up with the runway. This was only small left and right control deflections.  The flight deck had only minutes until landing.  What was the captain expecting to see, or not to see?  Can the captain after that brief manoeuvre be aware of his aircraft enough to make a decision?
Does this mean then that a Control Check only comes, or should only be done, if you are worried that the aircraft cannot land?  Can it be done at any other time?  In other words: is an in-flight control check only done for landing?

SOURCES:
Wikipedia
Flight 32:
https://en.m.wikipedia.org/wiki/Qantas_Flight_32

We do a flight control check at least twice before every takeoff.

@mins in my example. The joystick was moved left and right to simulate lining up with the runway. The captain wouldn’t have been able to tell all of your examples in such a short space of time would he?

@cmp You can tell if the airplane is moving pretty easily as you can see what changes out of the cockpit, plus there are plenty of instruments that you can look at to see which way the airplane is moving.

When you know the hydraulic system is compromised being able to land the plane safely or not isn't the first thing you should worry about. The pilot was probably worrying if he/she can still control the plane at all. In those very sad situations, when the pilot knows for a fact the plane could not land safely, evaluating the capability of controls is still useful because the pilot could use the remaining controls to "dispose" the plane in a way that minimizes collateral damage.

Fundamentally the job of the pilot is to control the plane, so for obvious reasons he needs to have constant awareness over the status of his control authority.

Are you asking specifically about a control-check during a flight? (This would most likely be called a "controllability check", BTW)

@UnrecognizedFallingObject would you mind elaborating in an answer please?

For what it's worth, the modern Airbus control sticks are usually referred to as the side stick, not a joystick.  I edited your question to clean up a few errors and put a few aviation related terms into your description of the control check.  Please take a look at it and edit the question again if the edit is not to your liking.

@KorvinStarmast many thanks for your useful edit.

Glad to be of help.

Technically speaking, a control check is part of any pre-flight.
In the case of Qantas 32, one engine had exploded, punching holes in the wing and setting off a host of warnings, so there was a reasonable expectation that some of the controls might also have been damaged.
What the pilot did was put the plane through some basic maneuvers to see how the controls responded in their now potentially damaged form. And found that roll rate had been affected, something he factored in on the subsequent landing. He also found that pitch control had not been affected, which meant he could make a reasonably conventional approach to the runway.
Better to find out how much the controls had been affected when there is time and altitude to recover from an unexpected response, than to find out on final approach, where the margin for recovery is very slim to none at all.
During flight, this is not normal. It is prudent if an inflight incident may have damaged some of the controls, so that the pilot knows the level of controllability they have to work with.

Why controllability checks matter
The procedure you're referring to from the QF32 mishap is called a "controllability check".  While a standard control check is done as part of a ground preflight to make sure that the flight controls work well enough to fly, a controllability check is an in-flight operation, performed either as part of a functional check flight or, more importantly, after the aircraft has sustained damage.
The controllability check is different from an ordinary control check in that while a control check is looking only for flight control motion, range of motion, binding, or free-play, a controllability check evaluates the airframe's response to control inputs in-flight at various speeds, typically from a stable clean speed at 10,000' down to a normal landing Vref if all is functional.  Flap and gear extension will also be tested if possible as part of this exercise due to their impact on aerodynamics.  Of course, if the controls start getting too close to full deflection during what basically is a mock landing approach, the pilots will knock it off and accelerate back to a safer speed, cleaning up the airplane if possible.
As a result of this, the pilots could determine what impact the damage had on airplane handling.  For instance, damage to a wing or wing control surfaces could have an impact in roll, while damaged or missing engines can be problematic at low speeds in roll and/or yaw.  Tail damage will often manifest itself as pitch control issues, but sometimes can be a yaw problem instead.  A load shift or other C.G. issue will also show up as pitch control trouble at low speeds.
A good example of a flight where one should have been performed was El Al 1862.  Had they performed a controllability check, they'd have noticed that the departure of engines #3 and #4 from the aircraft, as well as the loss of the right outboard aileron and right leading edge devices, had effectively jacked their Vmca up sky-high.  With this knowledge in hand, the crew could have recalibrated their expectations, shooting a partial flap approach at a much higher Vref speed to a choice of runway that gave them more favorable winds as they could still maintain flight at this higher speed and would need the headwind to help slow down after landing.

Aircraft control their flight using movable portions of the wing and tail called control surfaces; a control check is a check performed to make sure that they're capable of moving through the range of motion that they're expected to be capable of moving through. In the case of small airplanes, they're connected to the control column directly through the use of small cables, but in large airplanes, they use electrical sensors in the avionics to create electrical signals that then control hydraulics in the wings and tail that move the control surfaces.
In the case of the Qantas plane, the damaged engine might have damaged the control surfaces or the fluid lines that power the hydraulics, so they carried out the check to figure out how much functionality they had so that they could get an idea of how damaged the plane was, and how it would control as they came in for an emergency landing.


text:
How to calculate the altitude my aircraft normally loses in two minutes of gliding descent?
As this may be a rookie question, I just can't wrap my head around if it is an approximate or established number.
Doesn't it always depend on different conditions, such as wind(affecting AGL immensely) or temperature?
This is needed to determine the high key position for the 360 Forced Landing Pattern.
UPD:
I fly a Piper PA-38(best glide 70KIAS)
Once you open Flight Training Manual 4th edition on the page 129 you will find the following section - 360 Forced Landing Pattern.
I will write exactly as it goes:

"The turn is started with the aircraft heading in the direction of the
intended landing at an appropriate altitude over the desired touchdown
point. This starting position is called the high key. The
appropriate altitude depends on the glide perfomance of the aircraft.
To calculate this altitude, take the amount of altitude your aircraft usually loses in two minutes of gliding descent, add 200
feet, and you have the height above ground that works well for high
key. For example, if your glide rate is 600' per minute, the high key
altitude should be 1,400' AGL (2x600 + 200).

What I'm asking is why does the book state it in a way as if I knew my usual ROD every time I'm in the air?

How do I know precisely what the high key position for my aircraft and configuration is?
They are, in fact, making calculations after determining the high key and, therefore, I'm presuming that IT IS REAL and can be measured.

Apologies for this perplexing question. I should've probably made it clear from the beginning that its RoDx2 that I'm talking about.
I'm a novice and only two weeks into my flight training(I've had only one  flight), yet I'm reading everything in advance to perform better on my flights and make the best of it.  Appreciate all of you guys, you're being so helpful.

What information do you have?  (you can probably figure RODx2...)  Need glide ratio, speed, rate of descent, or some combination of numbers to determine the answer.  Also lift lost due to AOB.  Wind won't affect your rate of descent, but will affect your pattern.

The book states it as if I must know it somehow from the outset, even though the only number I have is the best glide(70 KIAS). It is definitely RODx2, but how do we determine it in the first place? I'm not really sure what the appropriate formula is.
I'm about two weeks into my flight training and it seems a bit confusing.

What aircraft are you asking about? Is there no chart for glide distance in the manual?

@UshbyTechnicalTeam, sure, yep.  You are correct.  Especially a glider in a thermal.  But I think you are overestimating the practical effect in an engine out situation.  But you might get lucky!

That is a question about the aircraft's sink rate at any given airspeed.  Sink rate is completely independent of horizontal wind component, and is only slightly affected by air temperature.  If you don't understand why sink rate is completely independent of horizontal wind component (excluding the cross over effect between horizontal wind component and vertical wind component due to slope lift, mountain wave lift, etc), then you need to study up, and maybe ask another question. --

check UPD section

@iconoclast Have you viewed my answer?  It shows you how you will calculate this using the information you do have available in the POH of any aircraft you fly.  In your case, the PA-38 glide chart is in Section 5 (Perfromance), whereas some manufacturers place it under Section 3 (Emergencies).  I will edit my existing answer, based on the 172N, by including the specifics for your aircraft at the end.

I can almost guarantee that in an emergency, you won't be making calculations.  Knowing wind speed and direction will be very important.  You need to make the field first.  Observe your glide angle and adjust.

I don't mean to be difficult, but this is in a lot of respects a "read the manual to me" kind of question.  I suggest that you go through the manual and look at all of the performance charts in it. That may shed some light.  Before you ever do an approach from high key for real you will have done one with your instructor so that you can see what it is that the book is talking about.  (I spent the better part of 4 years teaching the 'engine out' emergency landing profiles as a flight instructor, but not in this model of aircraft). Sometimes, the demonstration clears up misunderstandings.

In the case of the Gimli Glider, Wikipedia tells us: First Officer Maurice Quintal began to calculate whether they could reach Winnipeg. He used the altitude from one of the mechanical backup instruments, while the distance travelled was supplied by the air traffic controllers in Winnipeg, measured by the aircraft's radar echo observed at Winnipeg. In 10 nautical miles (19 km; 12 mi), the aircraft lost 5,000 feet (1,500 m), giving a glide ratio of approximately 12:1 so they computed it based on actual observations.

This will be much more intuitive if you move on to instrument training, where referring to the climb/descent rate gradient will seem like a daily thing.
POHs are different.  A 172N for instance, gives only a speed and a graph.  Others will give speeds calibrated by weight.  Some will give glide ratios.  But often in parametric physics equations, you have a few known values that you have to then convert into additional "intermediate" known values, that you only then can convert into the answer you need.
So, 172 as an example...  Best glide is 65, and I can tell by the graph that I'll cover about 1.5 NM per 1,000 feet of altitude.  Let's get to work!

1.5 NM = 9,000 feet, which means my glide ratio is 9:1, or expressed as slope is 1/9 (rise over run).  We don't need to know the degrees of slope, but if we're interested it works out using the tangent formula to be 6.3 degrees.

If I am moving at 65Kts (remember this is indicated, and both calibrated and GS will be different) then lets treat that 65 as horizontal motion.  Yes, technically it is the hypotenuse of a 1:9 triangle but the math required isn't justified by the fraction of a knot it will correct.  So at 65 Kts, I am covering about 6,500 FPM horizontally before compensating for wind.  Beyond a few thousand feet, miles are more useful than feet, so 6,500 feet is 1.083 NM.

I know from my glide ratio that if I am covering 6,500 FPM, a 1:9 ratio means I am descending at 720 FPM.  I can solve this just dividing 6,500 by 9, or I could have used the Pythagorean formula in step 2.  But at these glide ratios, division is accurate enough.

How long I will be aloft will therefore be AGL / 720, in minutes.  Or that answer times 60 if you need seconds.

How far I glide will be the time aloft (answer 4) x 6,500 feet, or if more useful in NM, it's time aloft x 1.08NM.

Now I have all the info I need to actually create a table, remmebering these estimates are in NO-WIND conditions.

AGL
Time mm:ss
Glide Dist NM
Glide Dist Ft

500'
0:42
0.75 NM
4,500'

1,000'
1:23
1.5 NM
9,000'

2,000'
2:46
3 NM
18,000'

3,000'
4:10
4.5 NM
27,000'

Remember!!!:

This is KIAS which at slow speeds will be slower then your KCAS, see your aircraft's calibration table in Section 5.  At 65Kts a 172 only has about 1-2Kts of error at most, but a 182RG could be 4 knots, and that's material to a glide plan.

Wind can make your groundspeed VERY different than KCAS, and in any ground-reference maneuver, you MUST take wind (and thus GS) into account.  And a gliding power-off landing is definitely a ground-reference maneuver, LOL.

This does not take into account the drag and load factors of a turn.  These are HUGE considerations in glide descent planning.  The POH assumption is always "land straight ahead, and you hit what you hit."

This does not take into account recognition time.  You can eat a lot of speed or a lot of altitude in just the few seconds it takes you to interpret the problem and commit to action.

This does not take into account your amount of rehearsal.  Recency of practice of power-off returns to field has been determined to be the single largest determinant of success, even for experienced pilots.

Finally, remember that any extra speed, altitude, or wind toward the field that is occurring at the key position increases your odds of inadvertently over-running the runway and going off the far end.  We get so focused on protecting the glide that we sometimes forget we still have to land.

Hope this all helps!
Adding some PA38 Specific information to better suit the OP
Here's a "typical" glide performance table from the PA38 POH.  Piper publishes this detailed table in section 5 (Performance) of your POH, whereas Cessna provides only a cursory graph in section 3 (Emergencies).

Some of the comments on this thread imply it's somehow a legal violation for you to have access to glide performance information, and I find those comments to be unacceptable and have stated that clearly.  The information is right in the POH, there's no secrets being revealed here.
You have to be careful about using this information only to form your own "intuition" about how your glide performance changes over a range of conditions, and not as a specific glide ratio you calculate at the moment of engine failure, as there won't be time.
What this chart shows is that at standard temperatures your glide ratio will be on the order of 7.5 to one, which is somewhat steep--which is what we'd expect to see with a glide speed of 70 Kts.
At 70 knots you are moving forward at roughly 7,000 FPM horizontally (no wind), and a 7.5 to 1 glide ratio means that you'll be descending in the neighborhood of 933 FPM.  That means that from 2,000 feet you'll be looking at just over 2 minutes aloft.  This is useful information to help think about what remedial actions I might have time to take, such as a restart attempt, but can't and must not be used for real "will I make the clearing" calculations.
This chart also tells me on a very hot day, say 10 deg. C above standard, I might see glide ratios more like 10.5 to 1, which is about 666 FPM vertically.  From 2,000 feet AGL I now have 3 minutes aloft and cover about 3.5 NM before wind.
This also means that on very cold days, your descent will be steeper at 70 KIAS, and your time aloft will be shorter and distance traveled will be as well.
I absolutely encourage you to spend some time with your instructor both at the white board and in the airplane to determine under what conditions you might adjust your glide speed based on weight and atmospheric conditions.  But that's not an exercise for strangers on the web.
Remember, there are many factors
The factors include weight, density altitude, load factors if turns are required, recency of practice, atmospheric conditions, And WIND, WIND, WIND, WIND.  The purpose of this exercise is to give you a mental picture, an intuition that serves as the starting point for your own thinking about how various atmospheric conditions affect glide performance.
As you move to other aircraft, the dynamics will change a lot and it's always worth spending some time with your instructor really digging into the performance tables and understanding "so what does this all mean."
Always, always strive to understand the factors that uniquely affect the performance of every single aircraft you fly, in every atmospheric condition that is common to your region.

check UPD section

Thank you for your great work! I will definitely spend some time with my instructor delving into all the subltle details I must understand about flying my aircraft. Cheers

Rate of descent is easy.  What you need for emergency landing high key is ground speed.
Because the aircraft "moves with the airmass" rate of descent for a given airspeed and (flaps) configuration does not change with wind conditions.  Ground speed does.
Your set up for an emergency landing can be practiced with gliding normal landings. (I combined my emergency and normal landing techniques due to limited flying at one (uncontrolled) field).
Landing without power enables one to practice glide angle control techniques by adjusting drag instead of thrust.  A Cessna 172 has 4 flap settings (0, 10, 20, 30) and can also be slipped.
So, when it really counts, come in a little high and set up to make midfield clean based on wind direction, then slip to shorten glide path a bit if needed.
In an emergency, it is way better to land long with 10 knots remaining than to crash short of the runway.

I covered mostly "the math" but Robert really provides excellent, must-read insights into the practice.  Hitting the trees at the far end with 15Kts left to bleed off is infinitely better than the near-end trees at 65.  Anywhere on the airport grass is still better than in a neighborhood, etc.  And every year we lose so many pilots because we fear the ground more than we fear loss of control.  There is no speed you can get down to that makes crashing inverted in a stall a better choice than crashing upright producing lift, PERIOD.  If I crash, I'm going to crash producing lift.

@UshbyTechnicalTeam  altitude loss covered in paragraph 2 and further more in your answer

check UPD paragraph under my question. Thanks for the answer!

Why calculate? in the pilot's manual for your plane it should list the descent rate in feet per minute for the plane when trimmed for best glide speed.
(feet per minute) x (2 minutes) = feet.


text:
Why do this airline experience more hard landings than others?
Ryan air has a reputation for hard landings, watching this video at the 2:22 mark and reading the subsequent comments (along with personal experience) suggests many people find the landing with Ryan air much less-smooth in general than other airlines such as Easyjet.
What is the reason for this, is it a difference in the software (i.e auto-land) or some difference between Boeing and Airbus that makes Boeing's harder to land smoothly?
Some entertaining comments from the video:

A smooth landing costs extra, like everything else on this plane.
Ryanair - "We pay for maintenance so we will use maintenance"
That music was to celebrate that the landing gear hadn't been completely destroyed.

NB: Their only entry in the Accidents and Incidents section of Wikipedia is for a non-fatal bird strike incident, so I'm not suggesting they are a dangerous airline to fly with.

Primarily opinion-based, voting to close.

Without data, there’s nothing to discuss. Could be anything from perceived customer base experience to real operations into on average shorter airfields requiring higher level of stopping performance (feels harder, but is simply better stopping)...

@CptReynolds The answer provided does provide some data though (length of landing gear), so we already have useful info from this question

"this video"  appears to show either a stall at about 5 feet or a marked reduction in power.  Aside from inclement weather (a possible wind gradient issue), and perhaps a shorter runway, yes, it could have been done better.

There is no evidence to support your assertion that Ryanair has a reputation for hard landings, you've extrapolated it from a single video.

"So whether there is an objective difference between Boeing and Airbus in difficulty of landing is an opinion?" Your question asserts that there is such a difference, and you then ask why. This is not the same as asking whether there is such a difference. Furthermore, this is a "Skeptics.SE" kind of question. While this does not make it off-topic here, I think it's not unreasonable to hold the OP to the same standards, which requires a notable claim (see their faq) for the question to be on-topic.

First of all, you are probably talking about a firm landing, not a hard landing as e.g. explained in this answer.
There is no evidence that Ryanair flights actually land more firmly than other airlines. Passengers are typically not very good at judging landings.
Also note that a firm landing is not a bad landing, it is in fact safer because it reduces required runway length!
Firm landings cannot be explained by auto-land software because the auto-land is only very rarely used (typically only when the conditions do not permit a CAT I ILS approach any more). More than 99% of all landings are hand flown.
A possible reason for a firmer touchdown feeling for Ryanair flights compare to easyJet is the shorter landing gear of the Boeing 737 (used by Ryanair) compared to the Airbus A320 (used by easyJet).
A more general reason for budget airlines touching down more firmly than other airlines is that they tend to use other airports with shorter runways, like e.g. Rome Ciampino (LIRA) instead of Rome Fiumicino (LIRF). A shorter runway means you have less room for error and cannot afford losing valuable runway to make a smooth touchdown.


text:
Can a commercial aircraft theoretically reach 60,000ft?
As per this question:

Altitudes above 60,000 feet are class E airspace.
That means that, if you are lucky enough to have an aircraft that flies that high, once you get there you could theoretically call ATC, cancel your flight plan and continue VFR above flight level 600 without ATC supervision.
At that point, you are on your own, and out of the system.
You are literally past the edge of human civilization.

If a commercial airliner, such as an A320 kept ascending, could it reach this altitude? If not, what is physically (ignoring legalities) stopping it from doing so?
Since it is not a legal requirement to use ATC at this altitude, perhaps this could be an airspace future budget airlines occupy to save time and therefore money.

I changed your title because class E in much of the US starts at 1200ft above ground, which commercial aircraft can certainly reach :-)

The highest flying commercial airliner produced was the Concorde which could reach altitudes of 60.000 feet in supersonic flight.

Just for your own fun, "service ceiling" is the specification you're looking for. Now obviously that's not necessarily the physical highest it can go, but the A320 is rated up to around 41000ft so you're unlikely to get another 50% out of it before you run out of lift.

Coffin corner is up above 41,000 feet for an A320, not something I'd want to play with.

@Dan Has one ever flown that high? Why would it need to?

An airliner's service ceiling is the altitude at which its rate of climb drops to less than 100 fpm.  Most commercial airliners can't climb above 40000-45000 on a standard day.  If temperatures are above standard, less than that.  Corporate aircraft can go higher, just above 50000, so they get to avoid pretty much all the traffic because everybody else is below 40.  The Global Express uses low bypass fans and very large wings to do this.
To go toward 60k you need super efficient engines and glider like wings.  P&W developed the U2's engine with virtually no blade tip clearance to be able to operate at 70000ft.
Then you have coffin corner, the spread between critical Mach# and stall.  It gets pretty small at 60000 ft.  In the U2 at 70000 ft the spread between Mcrit and stall is very small, and between Mcrit and the back side of the power curve is just a few knots and they cruise in a tiny speed window, autopilot on in speed mode at all times.  Bit too slow and down you go, bit too fast and mach buffet starts.
At an airliner's service ceiling, you have very little reserve thrust margin, and you may be operating just above min drag speed, not so far from the back side of the power curve in cruise.  If you are at service ceiling and some speed bleeds off because say you hit some down moving mountain wave and the autopilot pitched up to compensate, and you aren't paying close attention, you may have no choice to descend right away as speed decays with no way to stop the decay without resorting to gravity.
Stalls at this altitude take many 1000s of feet to recover from.  In the thin air, a mass has to accelerate over twice the distance through space for the same indicated airspeed increase as down low.  If you get stick shaker at that altitude, you have to push over really aggressively for the recovery, not just lower the nose a bit.  After a fatal Pinnacle Airlines CRJ200 stall incident that started at 41000, airlines started to focus more on training for high altitude stall recovery.
The high altitude world is a dangerous place, even at altitudes in the 30s.

Thanks for the answer John, but "even at altitudes in the 30s" seems a bit off, since that is where most commercial aviation happens, and we can establish that is not dangerous.

@Cloud as someone who primarily flies in them, I can assure you that the flight levels are dangerous. Factors include low temperature, low partial pressure of oxygen, and low air density. Concerns include losing pressurization, aerodynamic stall, and exceeding critical Mach, and exceeding engine limitations. We use technology and techniques to mitigate those dangers.

It's not "dangerous" when the limitations are observed by competent crews, but the limitations get tighter and tighter the higher you go.  And you can get into a lot of trouble even at 35000 ft.  A pilot without high altitude training can be dangerous.  Shortly after the Pinnacle incident we started getting sim training for shaker events and it was done at 35000 ft.   It was a minimum 3000 foot altitude loss just to recover from shaker (an RJ), let alone pusher or worse a real stall, because you need to pretty much dive to build enough speed to be able to pull up without hitting shaker again.


text:
Is there such a a thing as a single European pilot's license?
The pilot licensing process in Europe seems to be rather, well, complex... I've heard of JAA, JAR-FCL and now apparently EASA licenses, although EASA's website says otherwise:

the Agency is not authorised to issue pilot licences and therefore
  there will not be any EASA pilot licence in the future

And on top of that, individual countries can issue their own licenses (or at least ratings) that may not be valid in other countries.
So is there any such thing as a single European (EASA) pilot's license? What I mean by that is a single license that allows a pilot to operate any European registered aircraft in any European country, e.g a UK pilot flying an F-registered (French) aircraft in Germany?
There's some overlap with this question, but the answers there don't cover the practical use of a single pilot's license throughout Europe in the way I described.
EDIT: to clarify some comments, by "Europe" I mean EASA member states (let's keep it simple); and I'm asking about pilot licensing in general regardless of the type of flying. If there's any difference in where private/commercial/ATP license holders are allowed to operate in Europe that would be interesting to know.

By Europe I guess you mean EASA member states, not EU member states and not states or provinces geographically self identifying as European (e.g Kalingrad).

@RedGrittyBrick Yes, exactly

Is there ... a single license that allows a pilot to operate any European registered aircraft in any European country,

Yes.
According to the UK CAA
an EASA Part-FCL licence can be used to fly any EASA aircraft in any EASA member country.
I have interpreted your "Europe" as "EASA member" in order to avoid ambiguity about Switzerland, Croatia, Kalingrad, etc.

Most aircraft in Europe are EASA regardless of state of manufacture or registration.

The UK currently grants several levels of licence:

UK National Private Pilot Licence (NPPL)
UK Private Pilot Licence (PPL)
EASA Part-FCL Light Aircraft Pilot Licence (LAPL)
EASA Part-FCL Private Pilot Licence (PPL)
EASA Part-FCL Commercial Pilot Licence (CPL)
EASA Part-FCL Airline Transport Pilot Licence (ATPL)

hmm, AFAIK each country issues its own licenses, but they're recognised by all signatories. Effectively the same, but not quite a single license :)

@jwenting: That's true, I interpreted the "single licence" as being like the UK's single currency - where many banks can print and issue banknotes. There are many issuers of the EASA Part-FCL licences - but they all issue EASA Part-FCL licences and the paper document has a common format regardless of who issued it.

The light aircraft pilot's license (LAPL) allows leisure pilots to fly small aircraft in EU member states and is approved by the EASA. It is currently being implemented in member states, and will progressively replace national licenses. In fact, national licenses will cease to exist in the near future, in Europe.
This license allows its holder to act without remuneration as pilot in command in non-commercial operations on single engine piston aeroplanes-land or TMG with a maximum certificated take-off mass of 2000 kg or less, carrying a maximum of 3 passengers
There also is are variants of this license for helicopter pilots, ballons and for sailplanes (LAPL(H), LAPL(B) and LAPL(S)).
However, this is still a new regulation, involving several different national authorities, and is not yet fully in place. So if you are thinking of getting a pilot's license, you can choose to get the LAPL rather than your national private pilot's license. However, the deadlines for issuing national licenses and for converting licenses to the European standard keep being pushed back.

LAPL is a severely limited license and I don't think the original question was about that.

@PhilippeLeybaert I understand the question to be "Is there any such thing as a single European (EASA) pilot's license?". Yes there is: the LAPL, even though it still is a work in progress.


text:
Why does the use of reverse thrust on the DC-8 generate a pitch-up moment rather than a pitch-down moment?
Unlike most aircraft, the DC-8 is certified to use reverse thrust in flight (albeit only on its inboard engines). This serves as a very effective airbrake, although it also produces considerable buffeting (due to the large areas of separated and/or reversed airflow it produces over the wing’s surface).
It also, more interestingly, produces a pitch-up moment (at least according to the CAB AAR on the crash of EA304):

[...]  The pilots in this case, if the drive system failed during a PIO rather than earlier, had no way of knowing the real reason for its failure to operate in the ANU direction.  In the split seconds available to them for analysis they could easily have concluded that the failure was due to heavy stick forces.  Reverse thrust, in addition to drag, produces a nose up pitching moment, a fact known to the first officer if not to the captain, and as indicated previously, they did employ this aid.  It is also true that during the time, no matter how short, required to go from forward thrust to reverse, the noseup pitching moment of forward thrust has been removed and therefore contributes to the severity of the dive.  Small as it may be, this factor becomes more significant at very low initiating altitudes.  [Page 25 of the aforementioned AAR; my emphasis.]

For an aircraft with engines mounted below the aircraft’s center of mass, like the DC-8, only the use of forward thrust would be expected to produce a pitch-up moment (due to the torque exerted by the offset net thrustline), and reverse thrust should produce a pitch-down moment (for the same reason):

Yet, for the DC-8, the use of reverse thrust not only creates a nose-up pitching moment, but, apparently, creates one that is stronger than that produced by forward thrust!
Why?


text:
Why is the A380’s with-reversers stopping distance the same as its no-reversers stopping distance?
Thrust reversers, found on almost all jetliners1 and on the larger propliners, redirect engine thrust forwards, helping to slow the aircraft in the event of a landing or rejected takeoff.2  This takes some of the load off the aircraft’s wheelbrakes, reducing brakepad wear and tyre overheating, and considerably increases the total braking force available, allowing the aircraft to stop in a shorter distance than it otherwise would...3
...unless, that is, your aircraft is an A380, in which case, your reversers (all two of them), although they do reduce brake wear, apparently do not decrease the aircraft’s stopping distance (my emphasis):

The two reversers do help slow the A380—but not by much. In fact, unlike the thrust reversers on most airliners, including the Boeing 747 jumbo, they do not stop the aircraft in a shorter distance than brakes and spoilers alone. They do, however, take some of the strain off the brakes and are useful if water or snow makes the runway slippery.

How is this possible?  Any increase in available braking force should shorten the aircraft’s stopping distance (even if only by a small amount), so, assuming that the A380’s reversers produce a nonzero amount of braking force, the stopping distance for a given A380 at a given weight on a given runway under given conditions using maximum wheelbraking, maximum aerobraking, and maximum reverse should still be less than the stopping distance for said A380 at said weight on said runway under said conditions using maximum wheelbraking and maximum aerobraking, but no reverse thrust.
Yet, apparently, it isn’t.
Would someone like to explain this?

1: The main exceptions being the Fokker F28 and BAe 146, as well as early Caravelles.
2: A few jetliners, such as the DC-8, Il-62, and Concorde, can also use reverse thrust in flight, to steepen the aircraft’s descent angle without increasing its speed; in essence, the reversers are used as blown airbrakes.
3: This is especially important when wheelbrake effectiveness is degraded, due to, for instance, landing on a contaminated runway4 (which severely limits the amount of force that can be applied to the wheelbrakes without them locking up), with inoperative spoilers (which severely decreases the amount of weight carried by the wheels, and, thus, the amount by which their brakes can slow the aircraft), with one or more blown tyres (which renders the brakes on those wheel(s) useless for slowing the aircraft), overweight (which forces the aircraft to land at a much higher speed, where the coefficient of friction between the tyres and the runway is much lower than at lower speeds), with inoperative flaps and/or slats (ditto), with accreted ice and/or wing-leading-edge damage (ditto numero dos), with inoperative antiskid (which requires that the pilot(s) brake manually, and severely limit the amount of braking force [especially at high speeds] to prevent the wheels from locking up), with one or more inoperative wheelbrakes (for obvious reasons), numerous other things that I haven’t thought of, or any combination of the above.
4: For instance, one that is flooded (hopefully with water, not Jet A-1) or covered in (usually water) ice.

I suspect it would be different in case of an emergency braking

You're giving too much credence to one isolated statement by one reporter in one article. Typically, stopping performance is a matter of a selected deceleration rate - achieved this some brakes + reverse, of more braking & less reverse. Hence, a "no difference" case. I strongly suspect that author's statement wouldn't prove out with a dive into the A380's performance tables.

In all Airbuses auto-braking system aims for specific deceleration. Once the design target has been reached, the system does not apply more brake pressure although there usually is more available. So, if you apply thrust reverser with autobrake selected it will apply less pressure on wheel brakes and as the target deceleration remains the same, the stopping distance remains unaffected.
On slippery runways the autobrakes apply brakes until anti-skid starts to limit the brake pressure so in those conditions reverser would shorten the stopping distance. This would also apply to rejected take-off where maximum braking is commanded.

And this cannot be overridden by the pilot in case of an emergency, to brake faster, no matter the wear on the brakes?

@vsz why would locking up the wheels, or overheating the brakes and degrading their efficiency (possibly to zero), make the plane stop quicker?

@alephzero I think he means running the breaks at the limits (no locking/overheating), with maximum thrust through the reversers.

@vsz With autobrakes, no. But of course pilot can step on the brakes anytime and he’ll get full manual braking with anti-skid. Manual operation of brake pedals will disconnect autobrakes.

Adding to @alephzero's comment, I seem to recall a runway overrun accident in Norway which was caused by the pilot inadvertantly locking the wheels by applying the emergency brake when they felt that they wouldn't stop in time. The investigation eventually concluded that if the pilot had used only the normal brakes, they would have been able to stop on the available runway.

Rejected takeoff with trust reversers sounds like optimistic thinking.

@Joshua  Not if the reject is because an outboard engine (on the A380, anyway) failed!  Or due to a predictive windshear warning, or a vehicle on the runway, or a cargo fire warning, or etc...  Of course, the worst-case planning assumes no reverse during the RTO, but it's entirely possible that it may be available.

Autobrakes in max/RTO setting is maximum braking effort or "standing on the brakes". It doesn't aim for a specific deceleration in this mode.

The performance-targeting braking system is similar to torque demand engine control in modern cars. It's why a lot of "performance upgrades" seem to do nothing: the accelerator pedal controls torque demand, and upgrades only make such torque easier to achieve (e.g. better exhaust flow, cooler intake air), but there's no increase in overall torque limits or changes to accelerator-to-torque mapping. That's why tuning is necessary: to let the engine controller make use of higher available engine performance.

The A380 has an optional system called Brake-To-Vacate (BTV) which allows the flight crew to calculate required runway length and determine which runway exit they can make before starting descent. Once engaged, the pilot just has to put the aircraft on the runway and BTV takes over, applying spoilers and thrust reversers ahead of brakes to achieve the desired landing performance.
There's a more detailed look at the system here
Stopping performance for certification has to be determined without the use of thrust reversers, so the use of thrust reverser in service doesn't change that particular aspect of the aircraft's certified performance..

To clarify, BTV does not automatically deploy spoilers or thrust reversers. For those, a pilot action is always required. Indeed, BTV is a function of autobrake which in turn does not activate until spoilers are deployed. More over, it is true that thrust reversers are not taken into account when certifying performance on dry runways, but they do have effect.